The invention relates to a method for re-using silicon base material (S) of a metal insulator semiconductor (MIS) inversion-layer solar cell, where a first electrically conductive contact of full-area or partial type is arranged on one side of the base material, and the MIS solar cell-specific layers are arranged on the opposite side, said MIS layers being in the form of a first insulating layer (I) such as a silicon oxide layer, a second electrically conductive contact (M) provided on said first insulating layer and designed preferably in finger or lattice form, and a further insulating layer such as a silicon nitride or silicon oxynitride layer.
The worldwide development of photovoltaics for terrestrial use has been accompanied by continuous efforts to reduce the cost of producing solar cells. A major step forward was achieved with the MIS inversion-layer solar cell in particular, known from DE-PS 28 46 096, for example, whereby inexpensive polycrystalline solar cell silicon can be used, as this results in high efficiency with favourable manufacturing costs.
The base material of a corresponding cell comprises a mono- or polycrystalline p-conducting silicon. A first electrically conductive contact--partial or full-area--is deposited on the rear side. A silicon oxide layer of around 1.5 nm thickness as an insulating layer is deposited on the opposite side, preferably by a brief thermal oxidation process at relatively low temperature (approx. 500.degree. C.). The front contact, to be designated the second electrically conductive contact, is then deposited, and should have a finger or lattice structure. After provision of this contact the surface of the silicon oxide layer is coated with alkali ions, particularly cesium ions, in a subsequent immersion process in an solution containing alkali metal. A second insulating layer is then applied, preferably in the form of silicon nitride or silicon oxynitride. This is achieved with a plasma-assisted CVD (chemical vapour deposition) process at low temperatures (approx. 250.degree. C.) where a silicon nitride or silicon oxynitride layer approx. 80 nm thick is deposited that keeps the alkali ions such as cesium ions in their position. The necessary inversion layer is generated by influence in the silicon base material. The silicon nitride or silicon oxynitride layer doubles as an antireflection coating.
The mode of operation of the MIS inversion-layer solar cell model can now be described as follows.
Incident light between the areas forming the second electrical contact generates in the silicon base material electron-hole pairs which are separated in an electrical field. This field is generated by the presence of a stationary, positive surface charge at the nitride/oxide boundary layer, which induces in the silicon side facing the oxide a comparable charge of moving electrons and adjacent thereto a depletion zone. The layer having a thickness of approx. 10 to 50 nm with the moving electrons changes or inverts the conduction type of the silicon base material. The separated holes diffuse to the first electrically conductive contact, the rear electrode, while the electrons inside the conductive inversion layer wander to the second electrically conductive contact designed preferably in finger or lattice form, tunnel through the silicon oxide layer, and reach the external circuit.
Solar cells not conforming to specification--whether this is due to manufacturing method or ageing--are treated as rejects. It would however be desirable to be able to use such cells or at least parts of them further, so reducing the overall costs for manufacture of corresponding MIS inversion-layer solar cells.
It is therefore the object of the present invention to indicate a method for re-using silicon base material in an MIS inversion-layer solar cell which is both inexpensive and technically simple to implement.